/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        return recur(root) != -1;
    }
    private int recur(TreeNode root){
        if(root == null){
            return 0;
        }
        int left = recur(root.left);
        if(left == -1){
            return -1;
        }
        int right = recur(root.right);
        if(right == -1){
            return -1;
        }
        return Math.abs(left - right) <= 1 ? Math.max(left, right) + 1 : -1;
    }
}